A video purporting to show UK hostage Alan Henning being beheaded has been released by Islamic State militants.The Salford taxi driver was delivering aid to Syria in December when he was kidnapped and then held hostage by IS.
IS threatened to kill him in footage last month showing the death of Briton David Haines, and in this video they threaten US aid worker Peter Kassig.
David Cameron said Britain would do all it could "to hunt down these murderers and bring them to justice".
The prime minister said the killing of father-of-two Mr Henning, 47, showed "how barbaric and repulsive" IS was.
"My thoughts and prayers tonight are with Alan's wife Barbara, their children and all those who loved him," he said.
"Alan had gone to Syria to help get aid to people of all faiths in their hour of need."
Mr Henning's wife Barbara had this week appealed for her husband's release, saying: "He is innocent."
Volunteer Mr Henning was on his fourth aid mission to Syria when he was captured within minutes of arriving in the country last December.
IS has previously released videos showing the apparent beheading of two US journalists, James Foley and Steven Sotloff, and British aid worker Mr Haines.
The video released on Friday is yet to be verified, but it appears to show Mr Henning kneeling beside a militant, who is dressed in black, in a desert setting.
The footage ends with an IS fighter threatening a man they identify as Mr Kassig.
In a statement, Mr Kassig's family said he had converted to Islam and referred to him as Abdul Rahman Kassig.